Process and apparatus to calculate and measure the flow of a gaseous fluid

ABSTRACT

An apparatus to compute and measure the flow of a gaseous fluid by measuring the quantity of heat necessary to raise the temperature of the fluid comprising a sensing means disposed in the gaseous flow and a reference means disposed in a dead end cavity in such a manner as to be insensitive to the flow of fluid.

United States Patent 15 3,683,692 Lafitte Aug. 15, 1972 [54] PROCESS ANDAPPARATUS TO 3,085,431 4/1963 Yerman et a1 ..73/204 CALCULATE ANDMEASURE THE 2,591,195 4/ 1952 Picciano ..73/204 FLOW OF A GASEOUS FLUID3,433,069 3/ 1969 Trageser ..73/204 i 3,559,482 2/1971 Baker eta]...73/204 [72] Invent Rene Lam, 87 de Palm, 2,972,885 2/1961 Laub..73/204 Caen, Calvados, France [22] Filed: Jan. 7, 1971 PrimaryExaminer-Richard C. Queisser I2 I W No: 104,664 AssistantExaminer-Herbert Goldstein Attorney-Woodhams, Blanchard & Flynn I30]Foreign Application Priority Data [57] I ABSTRACT Jan. 8, 1970 France..7000516 An apparatus to compute and measure the flow of a gaseousfluid by measuring the quantity of heat neces- [52] US. Cl ..73/204 saryto raise the temperature of the fluid comprising a [51] Int. Cl ..G0lf1/00, GOlp 5/ 10 sensing means disposed in the: gaseous flow and a [58]Field of Search ..73/204 reference means disposed in a dead end cavityin such a manner as to be insensitive to the flow of fluid. [56]References Cited UNITED STATES PATENTS 10 Claims, 6 Drawing Figures3,326,040 6/ 1967 Walsh ..73/204 i 53 4 43 41 44 42 1 I 40 27 58 I Z? W25 29 11.1 32 f1 19 I 13 17 1a 31 26 l 1 g 14 14- 14 i mama Aug. 15,1972 2 Sheets-Sheet l fill m 7 V W .1 V m MMM Patented Aug. 15, 1972 2Sheets-Sheet 2 H% T f WW MM w M, 4 n

#2 H WM PROCESS AND APPARATUS TO CALCULATE AND MEASURE THE FLOW OF AGASEOUS FLUID The present invention concerns an apparatus to compute andmeasure the flow of a gaseous fluid by measuring the quantity of heatnecessary to raise the temperature of the fluid of a given quantity,comprising a sensing means disposed in the gaseous flow and a referencemeans disposed in a dead-end cavity in such a manner as to beinsensitiveto the flow of this fluid.

Such apparatus are known under the name of thermic flow meters and theyoffer, with reference to other types of flow meters, the advantage ofgiving an indication independent of the pressure and of the temperatureof the fluids, since the specific heat of a gas does not depend uponthese two factors. However, none of these apparatuses up to the presenttime has produced a truly satisfactory industrial embodiment,principally by reason of their complexity.

An object of the present invention is to remedy this inconvenience and,in order to do this, the invention provides an apparatus of theabove-mentioned type which is characterized essentially in that each of.the sensing means and the reference means comprise a heating resistor tocontinually heat the fluid in order to raise its temperature to a givenquantity and a detecting element sensitive to the temperature, thesensing means also including a further heating resistor, hereinaftertermed a heat compensating resistor, for which the current supply isregulated by the lack of balance between the two detection elements, formaintaining the said elevation of temperature of the fluid flowing pastthe sensing means, and means to continually measure the amount ofcurrent passing through this heat compensating resistance.

Thus, the apparatus is practically insensitive to variations in theambient temperature, since the supply of current to the heatcompensating resistor is controlled by the difference of the temperatureexisting between the sensing means and the reference means which is notsubject in the fluid flow. Moreover, as the fluid flow increases, theheat supplied by the heat compensating re sistor increases, so that thetemperature of the sensing means remains identical to that of thereference means. The dissipation of electrical energy in this heatcompensating resistor is thus really proportional to the fluid flow.

It can likewise be noted that the phenomena set in action being purelythermic, the apparatus is comprised of no movable parts and it is thusof great strength and practically unbreakable. It is easily seen, inaddition, that such an apparatus would be insensitive to water hammer orto accidental peak flows.

In a particular embodiment of the invention in which the sensing meansand the reference means are both incorporated in a measuring head, ofwhich the body is fixed to a divider block of tubular flow, insertedinto the pipeline through which passes the gaseous fluid for which onedesires to measure the flow, this flow divider block communicating witha measuring head, on the one hand by an entry port and on the other handby a discharge port by which the gaseous fluid proceeding from themeasuring head rejoins the principal flow, the entry port of the gaseousflow proceeding into the measuring head is furnished with calibrateddischarge pipe, whereas the flow divider block comprises between the 2intake opening and the corresponding discharge opening, a removabletransverse plate provided with calibrated perforations.

Due to this assembled arrangement, large flows are measured by the meansof a fraction of the principal flow which is directed toward themeasuring head. One can thus measure, with a single measuring head,widely varying flows by simply modifying the ratio between the passagesection of the outlet pipe and of the perforated plate.

Advantageously, the flow divider block comprises moreover a perforatedgrill and a screen, disposed transversely upstream of the intake openingof the gaseous flow proceeding into the measuring head.

The perforated grill has for its object to equalize the eventualturbulence of the gaseous flow, while the screen subdues theseturbulences and stops the large particles which could be found insuspension in the gas.

The reference means and the sensing means are essentially comprised oftwo identical metallic probes in the form of bells, each comprising ahole in which are respectively disposed, in a metallic alloy, theheating resistor and the detecting element for the reference means andthe heating resistance, the detecting element and the heat compensatingresistance for the sensing means, an artificial resistance, of the samenature as the heat compensating resistance of the sensing means beingmoreover immersed in a corresponding hole of the probe of the referencemeans, in such manner that the loss of heat by thermic conduction shouldbe identical for the two means.

The intake opening of gaseous flow derived empties into a practicallyannular chamber in the body of the measuring head, a calibrated outletpipe equipping the input opening and being of very reduced diameter sothat the gaseous flow derived ought to be actuated by the turbulentmovement into the cavity of the said chamber.

Moreover, a hollow ring, thermally insulated, is disposed in theinterior of the annular chamber, so that the fluid takes on thetemperature of this ring, which ring changes temperature with the samethermal phase shift as the probe of the sensing means.

The cavity of the ring communicates at its inner part with a firstcavity in the recess in which is disposed the reference means and with asecond similar cavity in which is disposed the sensing means, thissecond cavity communicating moreover at its upper part with a conduitjoined directly with the discharge port of the derived gaseous flow.

Thus, the gaseous flow has time to reach the ambient temperature of themeasuring head before entering into the interior of the cavity in whichis located the sensing means.

Advantageously, the interior walls of the two cavities I trols thefunctioning of a monostable multivibrator supplying the heatcompensating resistor of the sensing means, the means which permitcontinuing measurement of the current traversing the heat compensatingresistance being constituted by a milliammeter connected in parallelwith said resistance by means of a filter circuit.

Thus, the intensity of the unbalanced current in the bridge determinesthe frequency of changing the state of the monostable multivibrator and,consequently, the frequency of the impulses which are delivered by thismultivibrator to the heat compensating resistor. Moreover, theunbalanced current will be intense and further the frequency will beelevated, which produces in the heat compensating resistor a currentintensity average much greater, and thus a larger heating of the latter.Besides, due to the filter circuit, the current which passes in themilliammeter is practically continuous and it is proportional to thefrequency of the impulses delivered by the monostable device, and thusproportional to the heating power of the heat compensating resistorwhich is supplied by the same impulses.

Preferably, the apparatus comprises moreover an impulse counter,controlled by the monostable multivibrator and acting as a countaccumulating device registering an integration of the fluid flow.

The counter has a consummation equivalent to that of the heatcompensating resistor and is supplied in phase opposition with regard tothat resistor, so as to assure a constant drain on the power supply,regardless of the fluid flow.

The counter registers thus the total quantity of gas which passesthrough the measuring head, by totalling the number of impulsesdelivered by the monostable multivibrator. MOreover, as it is suppliedin phase opposition, and it is arranged so that it consumes very nearlythe same current as the heat compensating resistance, the currentdelivered from the supply will be practically constant, for contributingthus to a good stabilization of a regulated voltage.

One embodiment of the invention is described hereinafter by way ofexample and with reference to the attached drawings in which:

FIG. 1 is a sectional view of an apparatus according to the invention,in which is schematically shown the case containing the electroniccircuit for measuring which is associated with this apparatus;

FIG. 2 is a sectional view taken along the line II-II of FIG. 1;

FIG. 3 is an enlarged, fragmentary, sectional view taken along the lineIII-III of FIG. 2;

FIG. 4 is an enlarged, fragmentary view taken along the line IVIV inFIG. 2;

FIG. 5 is a schematic diagram of the electronic circuit of measurementassociated with the apparatus according to the invention; and

FIG. 6 is a diagram representing, as a function of time and amplitude,the direction of the potential at a point in the circuit of FIG. 5, fortwo different values of the flow of fluid measured.

The apparatus represented in FIGS. 1 and 2 is comprised principally of aflow divider block 1 and of a measuring head 2 which is attached by anelectrical cable 3 to a case 4 containing the associated electronicmeasuring circuit.

The flow divider block 1 is essentially comprised of a tubular metallicbody 5 which is inserted into a pipeline 6, through which passes thegaseous fluid for which it is desired to measure the flow, by means offlanges 7 on which support is taken by the bolts 8. Sealing between thebody 5 and the flanges 7 is secured by two O-rings 9. As to themeasuring head 2, it is essentially comprised of a cylindrical, metallicbody 10 which is affixed by its rectangular base to the body 5 of theflow divider block 1 by means of four screws 12, a seal being assured byan O-ring or sealing element 13.

The gaseous fluid flows through the pipeline 6 in the directionindicated by the arrows l4 and first encounters successively aperforated grill 15 and a metallic screen 16, mounted in the interior ofthe tubular body 5, and transversely thereof. The gaseous fluid thendivides into two different circuits. One small part of this fluid isdirected into the measuring head 2, as indicated by the arrows 17 inFIG. 1, by means of an entrance opening 18 provided in the wall of thebody 5 and which is furnished with a calibrated delivery pipe 19. Therest of the fluid, which constitutes the major part thereof, passesdirectly through the flow divider block 1 by means of a plate 20 havinga certain number of calibrated perforations 21. This perforated plate 20is mounted in a removable manner in the interior of the tubular body 5,in the central part thereof. As indicated by the arrow 22, the gas flowproceeding through the measuring head 2 rejoins the principal flow whichpasses through the flow divider block 1, by a discharge opening 23constructed in the wall of the tubular body 5 and emptying into theinterior thereof on the other side of the perforated plate 20. In theinterior of the cylindrical body 10 and the measuring head 2 is arranged an annular, peripheral chamber 24 into which discharges theopening of the calibrated delivery pipe 19. In this annular chamber islocated a hollow ring 25.

Between the body 10 and the divider 1 is located a therrnic isolatingdisk 26, for example, of plastic material, having some openings assuringthe passage of the fluid therethrough.

In the central part of the body 10 are also arranged two cylindricalcavities 27 and 28 containing respectively a sensing means or element 29and a reference means or element 30. The cavity 27 communicates at itslower part with the annular chamber 24 by the opening 31 andcommunicates at its upper part with a passage 32 connected to thedischarge opening 23. The cavity 28 is a cavity in recess and onlycommunicates with the annular chamber 24 by the single lower opening 33.

The sensing means 29 which is represented in a larger scale in FIG. 3,is essentially comprised of a metallic probe 34 having in its lower parta cavity 35 in the form of a bell, the probe is thermally isolated fromthe walls of the cavity 27 by means of a ring 36 and a disk 37, formedfrom plastic material. The ring 36 is furnished with a passageway 38 ona level with and communicating with the conduit 32, whereas the disk 37is furnished with a passageway 39 adjacent and communicating with theopening 31.

At the upper part of the probe 34 is arranged an axial opening in theinterior of which are immersed, for example, in a metallic alloy havinga low point of fusion, a miniature thermistor Th 1, a miniatureresistance R1,

and they are electrically connected to the remainder of 5 the circuit,contained in the measuring case 4, here by means of a small printedcircuit plate 40 including terminals 41 to which are soldered theextremities of corresponding conductors 42, constituting the connectingcables 3. This printed circuit plate 40 is disposed on the top of thebody and it is protected from external influences by a cover of plasticmaterial 43 which is threadedly engaged with the periphery of thecylindrical body 10 while completely enclosing it. The connecting cable3 passes through the cover 43 by means of an opening provided for thispurpose in the upper part of the cover and which is equipped with aninsulating grommet 44.

The reference means 30, which is shown in enlarged scale in FIG. 4, isessentially comprised of a metallic probe 45 in all respects identicalto the probe 34 of the sensing means 29. This probe is also thermallyisolated from the walls of the cavity 28 by a ring 46 and a disk 47 madefrom metallic material. The disk 47 is furnished with a passageway 48adjacent to the opening 33, whereas the ring 46 is deprived of allpassageways, being in the dead-end cavity 28.

At the interior of the axial opening provided in the upper part of theprobe 45 are inserted a miniature thermistor Th 2, a miniature resistorR2, called the constant heater, and a provisional resistor P. Thethermistor Th 2 and the resistor R2 are also necessary parts of theassociated electronic circuit and they are connected electrically intothe measuring case 4 by means of the abovementioned printed circuitplate 40. The provisional resistor P is short-circuited and thus I playsno role in the functioning of the associated electronic circuit. Itsimply occupies the place of the resistance R of the sensing means 29and has for its purpose to render the reference means 30 perfectlyidentical to the sensing means 29 from the standpoint of geometric andthermal identity.

The associated electronic circuit, of which the schematic is shown inFIG. 5, is comprised principally of a stabilized supply 49, of a circuit50 comprising a measuring bridge associated by an amplifier withtransistors of a circuit 51 comprising a monostable multivibrator, tworesistor of a measuring circuit 52 associated with a milliammeter 53, ofa recording circuit 54 associated with a recorder 55 and of a controlcircuit 56 for an impulse counter 57.

The stabilized supply 49 is of a known type and comprises first atransformer Tr on the primary side of which is applied the alternatingcurrent of an A.C. source. The secondary coil of this transformer has acenter tap M constituting the zero or reference level of the supply andits two extremities are respectively connected to two diodes D1 and D2for providing full-wave rectified current. The stabilized supply 49further includes a filter condenser C1, two Zener diodes Z1 and Z1connected respectively in series with the resistances R3 and R4 acrossthe condenser C2, and a regulation transistor T1, of the type NPN,connected as the supply output device. The positive potential, E,appearing on the emitter of transistor T1 is thus perfectly stabilizedand constitutes the supply voltage of the complete electronic circuit.

In the circuit 50 is found first of all the resistance R1 and thethermistor Th 1 of the sensing device 29, as well as the resistance R2and the thermistor Th 2 of the reference device 30. Each of theresistors R1, R2 is directly connected between the emitter of thetransistor T1 and the midpoint M, that is to say in parallel with thesupply. As to thermistors Th 1 and Th 2, they are connected in seriesacross the supply. These two thermistors are of equal value andconstitute two adjacent branches of a Wheatstone Bridge. The two otherbranches of the bridge are determined by a voltage divided supplyfurnished on the potentiometer P1 which is connected across the supplyby means of three resistances R5, R6 and R7 in series therewith.

The junction point B of thermistors Th 1 and Th 2 is connected to thebase of a transistor amplifier T2, of the type PNP, of which the emitteris connected to the voltage divider supply ultimately furnished bytransistor T1. The deviation, due to temperature, of this transistor T2is compensated by a thermistor Th 3 connected in parallel with theresistance R6.

The collector of transistor T2 is connected directly to the base of asecond transistor amplifier T3, of the type NPN, of which the collectoris connected to the potential E by a fixed resistance R8, and of whichthe emitter is connected to the center tap M by a resistance R9. Aresistance R10 is also connected between the base and emitter of thistransistor T3.

The monostable multivibrator which comprise the circuit 51 isessentially comprised of two transistors T4 and T5,.of type NPN, ofwhich the emitters are directly connected to the zero or reference levelof the supply. The base of transistor T4 is connected to the emitter oftransistor amplifier T3 by means of a resistance R11, whereas itscollector is connected to the base of transistor T5 by a resistance R12in series with a condenser C2. A condenser C3 is also connected betweenthe collector of transistor T5 and the base of transistor T4.

The collector of transistor T4 is moreover connected to potential E ofthe supply by means of a potentiometer P2 in series with the heatcompensating re sistor R of the sensing means 29. As to the collector oftransistor T5, it is connected to the potential E by a single resistanceR13. Finally, the base of T5 is con nected to potential E by apotentiometer P3.

The measuring circuit 52 first comprises three resistances R14, R15 andR16, connected in series between the one of the posts of milliammeter 53and the junction point G of the resistance R with the collector oftransistor T4. The junction point of resistors R14 and R15 is connectedto potential E by a filter condenser C4, whereas the junction point ofresistor R15 and R16 is connected to this same potential E by a filtercondenser C5. The point G is moreover connected to an extremity of onediode D3 of which the other extremity is connected to potential E by acondenser C6 and to condenser C5 by a resistance R17. The other post ofmilliammeter 53 is connected to potential E by means of a resistance R18which in reality is a part of the recording circuit 54. In fact, therecorder 55 is connected at one input to this resistance R18 and itsother input to a divided voltage supply H connected to potential E by aresistance R19 and to the zero or reference side of the supply by aresistance R20.

The command circuit 56 is essentially comprised of a transistor T6, oftype NPN, of which the base is connected to point G by means of aresistance R21. The collector of this transistor is connected directlyto potential E of the supply, the impulse counter 57 being connectedbetween the emitter thereof and the zero side of the supply. Themilliammeter, which as one sees by the following constitutes the flowindicator means of the apparatus, is mounted on the front face of case4, as shown in FIG. 1. Likewise, there is provided a window 58 on thefront face of the case 4, in order to permit the appearance of theindications of the impulse counter 57 which totals the total quantity ofthe gas consumed.

The apparatus which has been described functions in the followingmanner:

The gas fluid, which moves along in the interior of the pipeline 6 inthe direction indicated by the arrows 14, meets first of all theperforated grill of which the object is to stabilize the eventualturbulences of the fluid. It next meets the metallic screen whichdiminishes these turbulences and assures at the same time a filtering oflarge particles likelyto be found in suspension in the gas.

Just as has already been said, the gaseous fluid then divides into twodifferent circuits. The major part of the fluid passes directly throughthe flow divider block 1 by means of the perforated plate 20, while asmall part of it is directed into the measuring head 2 by the calibrateddelivery pipe 19. The quantity of gas directed into the measuring headdepends essentially upon the ratio between the area of the passage ofthe delivery pipe 19 and the total area of the calibrated perforations21 in the plate 20. It is sufficient thus to measure the directed flowin order to have the value of the total flow. It is noted moreover thatthis plate is removable, thus permitting adaptation of the flow dividerblock to function in nominal flow of the pipeline 6.

The gaseous flow which enters into the measuring head 2 departs from thedelivery pipe 19 in a turbulent fashion and circulates firs in theinterior of the ring 25 situated within the annular chamber 24. Thispermits the gas to obtain the ambient temperature t1 of the assembly ofthe measuring head 2 before entering the interior of the cavity 27 ofthe sensing device 29 by means of the passages 31 and 39. The fluid thencomes swirling into the cavity35 of the probe 34, fills the annularspace between the probe and the ring 36 and discharges again through thepassage 38 to go and rejoin, by means of the passage 32 and thedischarge orifice 23, the principal flow which passes through the flowdivider block.

The permanent heat resistance R1, being connected in parallel with thesupply, causes the probe 34 to assume at rest, that is to say, in theabsence of gaseous flow, a temperature t2 when the assembly of themeasuring head is at the ambient temperature t1. The difference t2 :1only depends upon the heating power of the resistance R1 and as thisheating power is constant, the difference 12 t1 is likewise constant.

The resistance R2, also being connected in parallel with the supply,heats thus also constantly the probe 45 of the reference device 30. Theprobe 45 finds itself, consequently, at the same temperature t2 as theprobe 34.

The flow of gas which passes through the measuring head 2 tends to coolthe probe 34 in passing through the cavity 27. On the other hand, theprobe 45 is not cooled since the cavity 28 in which it is located is adead-end and thus does not have the gas flow therethrough. The loweringof the temperature of probe 34 by reference to that of the probe 45 isdetected by the thermistor Th 1 which controls then, as will be seen bythe following, the passage of a certain current in the heat compensatingresistance R. This current is such that the heat developed by resistor Rreturns the probe 34 to the temperature t2. Otherwise stated, thedifference of temperature t2 t1 is maintained constant, despite the flowof gas through the measuring head.

The gas entering the cavity 27 at the temperature t1 is heated to thetemperature :2 and thus accumulates a quantity of heat which is thefunction of its mass or of its flow rate. This quantity of heat isbesides proportional to the electrical power dissipated in theresistance R and it is thus proportional to the current flow throughthis resistance since the latter is delivered by impulses at constantvoltage, as one will see by the following. It is sufficient,consequently, to measure this current by means of milliammeter 53provided for this purpose, to have the instantaneous value of flow ofgas delivered into the measuring head. Moreover, as the delivered flowis proportional to principal flow, it is sufficient to calibrateconveniently the milliammeter 53 in order that the latter gives directlythe instantaneous value of the total flow of gas passing through thepipeline 6.

However, in order that the ratio between the delivered flow and theprincipal flow should be perfectly constant, it is necessary that thedelivery pipe 19 be sufficiently small that the gas pressure drop whichit causes is large by comparison to that caused by the remainder of thepath through the measuring head.

It is likewise convenient to notice that the gas never has time torigourously assume the temperature t2. In fact, the gas assumes atemperature which depends theoretically on the flow. Moreover, the gashas two possibilities of advancing through the cavity 27. For the smallflows, the gas flows in a laminar discharge which goes directly out bythe passage 38 without coming to whirl in the cavity 35 of the probe.The surface of heat exchange is thus smaller and this compensates thetemperature for a time of contact which is relatively long. On the otherhand, for the large flows, the gas has a turbulent discharge and swirlsinto the cavity 35 before flowing out to the passage 38. The surface ofheat exchange is thus greater and this compensates for a time of contactwhich is relatively short. There is ob tained thus a linear responsebetween the value of flow and the indication given by the milliammeter53. Moreover these two possibilities of advancement of gas through thecavity 27 permit compensation for the slight distortions due to thedissipation of heat of the sensing means 29 which is not precisely thesame in repose and circulation of gas.

In the absence of the hollow ring 25, the gas is able to take thetemperature t1 simply by contact with the annular chamber 24. Thepresence of the hollow ring 25 becomes useful following periods wherethe measuring head 2 changes temperature. In this case, the probe 34follows the changing of temperature with a phase shift in thetemperature as a result of its thermal isolation. This hinderingphenomena is compensated by the fact that the gas in penetrating intothe head 2 takes the temperature t1 in the interior of the ring 25 ofwhich the mass and thermal isolation are such that its temperaturechanges with the same phase shift asthe probe 34.

The operation of the associated electronic circuit, referring moreparticularly to the schematic of FIG. 5, will now be explained. As onesees from the following, the transistor T1 of the supply 49 functions ata constant voltage. It can thus be considered that the positivepotential E of its emitter is perfectly stabilized.

The reference means 30 is geometrically and thermally perfectlyidentical to the sensing device 29, notably by the presence of theprovisional resistance P in the probe 45, at the place of the resistanceR of the probe 34. Consequently, the middle point B of the connection ofthe bridge constituted by the two therrnistors Th 1 and Th 2 is notaffected by variations of the temperature of the assembly or themeasuring head. In fact, if the ambient temperature varies, for exampleincreases, the temperature of the sensing device increases in the sameproportion as that of the reference device and the bridge remains inequilibrium. The potentiometer P1 permits the adjustment to zero of theoutput of the bridge at rest.

As indicated above, the passage of a flow of gas through the cavity ofthe sensing device 29 tends to cool the probe 34. Consequently, theresistance of the thermistor Th 1 varies from that of the thermistor Th2 of the reference device 30, which causes an unbalance of the bridge.The unbalanced current of the bridge is at first amplified by thetransistor T2 for which deviations in temperature are compensated by thethermistor Th 3. This current is thereafter amplified by the secondtransistor T3.

The resistors R9 and R10 have for their purpose the maintenance in thetransistor T2 of a collector current not negligible in repose, whichpermits this transistor to work with a coefficient of amplificationsufficiently elevated. As to the resistances R8 and R11, they operate ascurrent limiting resistances to prevent damage to the transistor 23 inthe case of a great unbalance of the bridge. The unbalanced current ofthe bridge, amplified by the transistors T2 and T3, controls then thefunctioning of the monostable multivibrator of circuit 51.

This monostable multivibrator, constituted by the transistor T4 and T5,is in a stable state when the potential of the base of T4 is negative ornear zero. In this case, the transistor T4 is in a nonconductive stateor blocking. The current in the heat compensating resistor R is null andthe potential of point G is thus equal to E. The transistor T5, of whichthe base is supplied by the potentiometer P3 is thus conductive andmaintains in the resistance R13 a current which is such that thepotential of its collector is very low.

When the bridge is unbalanced, the current amplified by the transistorsT2 and T3 increases the potential of the base of T4 and the base currentof the latter, amplified, commences to lower the potential of point G.This lowering of potential is transmitted to the base of T5, by means ofthe condenser C2, and it causes an increase in the potential of thecollector of this transistor. This last increase of potential is alsotransmitted to the base of T4 by the condenser C3 and entails bycumulative effect, unblocking of transistor T4 and blocking oftransistor T5. The resistance R thus provides its maximum voltage dropE, assuming that the potentiometer P2 is set at zero resistance. Withthis changing state of the multivibrator, the condenser C2 will haveaccummulated a negative charge E and the collector of transistor T5 willbe raised to potential E.

This state, called unstable, lasts the time of the discharge ofcondenser C2 into the potentiometer P3. After that, the transistor T5begins again to conduct through the resistance R13, which causes areduction of the potential of its collector. This reduction of potentialis transmitted by C3 to the base of transistor T4 which then findsitself once again blocked and the multivibrator returns again to itsinitial, stable state. This cycle recommences as soon as the currentissued from transistor T3 has discharged the condenser C3.

One sees thus that the resistance R will pass a series of currentimpulses, the frequency of which will increase with an increase in thecurrent issued from transistor T3, that is to say, as the unbalance ofthe bridge increases. FIG. 6 represents, by way of example, the changeof the potential at point G of circuit 51, as a function of time and interms of its amplitude A. The curve 59 (FIG. 6) represents the change ofthis potential in the case of a small flow and! the curve 60 in the caseof a large flow. Thus, there is plotted a series of negative impulses61, the amplitude of which is equal to E and the frequency of whichincreases with the flow. The duration of these impulses is determined bythe time constant C2, P3.

The voltage impulses appearing at the point G of the circuit 51 creates,in the circuit R14, R15, R16, 53, R18, a certain current which, filteredby the condensers C4, C5, give in the milliammeter 53 a useful currentpractically continuously proportional to the frequency of theseimpulses. The heat dissipated by the heat compensating resistance R isalso proportional to this frequency. The indications of the milliammeter53, conveniently calibrated, represent the instantaneous value of thegas flow passing through the pipeline 6.

In fact, in the assembly shown, weak flows result in a slightcontraction of the scale The latter is compensated by the diode D3which, under the impulses of G, charges capacitor C6 which rapidlyattains the charge E to remain blocking at this value for the higherflows..The condenser C6 in discharging slowly into re sistances R17 andR16, introduces into the milliamme ter 53 a correction which is thusparticularly apparent for weak gas flows.

The potentiometer P2 permits, at the time of calibration of theapparatus, compensation for small differences which may exist betweendifferent measuring heads 2 or connecting cables 3. This also permitsadaptation of the apparatus to a gas for which the thermal conductivitywould be conditioned to a different varia tion of t2 t1.

The voltage impulses appearing at point G of the circuit 51 areamplified by the transistor T6 of the control circuit 56. Thistransistor is connected in the common collector mode and the impulsecounter 57 is connected to the emitter thereof. Under these conditions,the output of transistor T6 is in opposite phase to that of transistorT4 of the monostable multivibrator. In consequence, as the counter 57consumes practically the same current as the compensating heaterresistor R, the total current delivered by the stabilized supply 49 willbe practically constant, which contributes to a perfect stabilization ofthe regulated voltage E.

The counter 57 registers the number of impulses appearing at the point Gand and totals also the total quantity of gas traveling through thepipeline 6. The potentiometer P3 determines the duration of theseimpulses and thus allows adjustment of the indications of the counter57as a function of those given by the milliammeter 53.

The current passing through the milliammeter 53 gives, at the terminalsof the resistance R18, a voltage capable of controlling the counter 55.However, it is seen that this recording is not absolutely indispensable.In the case where it is used, it may be provided for in the measuringcase 4 by a flexible cable provided for this purpose.

In fact, the base current of transistor T6 causes in the resistance R asmall lowering of voltage drop which distorts the zero point. For themilliammeter 53, the latter can be adjusted for by resetting to zeromechanically. For the recorder 55, the zero point may be adjusted inselecting its control voltage, not only on the resistance R18, but alsoon the supply to the potentiometer H, for which the potential is verynear to E for the value of resistor R19 is very small, compared to thatof resistor R20.

Although a particular preferred embodiment of the invention has beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modifications of the disclosed apparatus, includingthe rearrangement of parts, lie within the scope of the presentinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Apparatus for counting and measuring the flow of a gaseous fluid in apipeline by measuring the quantity of heat necessary to raise thetemperature of a given quantity of the fluid, comprising a sensing meansdisposed in the gaseous flow and a reference means disposed in adead-end cavity containing the gaseous fluid in a manner to be sensitiveto fluid temperature but insensible to the flow of this fluid, in whicheach of the sensing means and reference means comprises a heat resistorfor continually heating the adjacent fluid in order to elevate itstemperature to a given quantity and a detection element sensitive to thetemperature, the sensing means comprising also a heat compensatingresistor, means responsive to a difference between the temperatures ofthe two detection elements for controlling the amount of current passingthrough said heat compensating resistor so as to cause the heatcompensating resistor to maintain the said elevation of the temperatureof the fluid flowing past said sensing means, and means for measuringcontinually the amount of current passing through said heat compensatingresistance.

2. Apparatus according to claim 1, including a measuring head having abody, in which the sensing means and the reference means are bothdisposed in said measuring head, a tubular flow divider block insertedinto the pipeline through which flows the gaseous fluid for which it isdesired to measure flow, said flow divider block being fixed to themeasuring head body and communicating with the measuring head, on theone hand by an entry orifice for applying fluid to the measuring headand on the other hand by a discharge orifice by which the gas flowdirected into the measuring head rejoins the principle flow, the entryorifice being furnished with a calibrated port, a removable transverseplate provided with calibrated perforations located in the flow dividerblock between the entry ori fice and discharge orifice.

3. Apparatus according to claim 2, including means defining an annularchamber in the body of the measuring head, the entry orifice of saiddirected gas flow opening into said annular chamber, said gas flowing insaid annular chamber before reaching said sensing and reference means,the calibrated port associated with the entry orifice being very smallso that the directed gas flow is agitated to a turbulent movement insaid chamber.

4. Apparatus according to claim 3, including a hollow ring, thermallyinsulated, disposed in the interiorof the annular chamber, so that thefluid takes the temperature of this ring, said ring having the samethermal phase shift as the probe of the sensing means.

5. Apparatus according to claim 4, in which the cavity of the ringcommunicates at its lower portion with a first dead-end cavity in whichis disposed the reference means and with a second similar cavity inwhich is disposed the sensing means, this second cavity alsocommunicating at its upper portion with a conduit connected directly tothe delivery orifice of the directed gas flow.

6. Apparatus according to claim 5, in which the inten'or walls of thetwo cavities, in which are respectively disposed the reference means andsensing means, are coated with a thermal insulating material, such asplastic material.

7. Apparatus according to claim 1, including a measuring headincorporating said dead end cavity and further incorporating a throughflow cavity, in which the reference means and the sensing means areessentially comprised of two identical metallic probes each in the formof a bell, said reference means probe being located in said dead endcavity and said sensing means probe being located in said through flowcavity, said bells each having a recess containing a metallic alloy intowhich are respectively received the heating resistor and the detectingelement for the reference means, and the heating resistor and thedetecting element and the heat compensating resistor for the sensingmeans, a provisional resistance, of the same nature as the heatcompensating resistor of the sensing means, being also inserted into therecess of the probe of the reference means.

8. Apparatus according to claim 1 in which the two detecting elementssensitive to the temperature of the gas flow are comprised ofthermistors, said difference responsive means including means connectingsaid thermistors in two adjacent branches of a Wheatstone bridge, amonostable multivibrator supplying the heat l4 tivibrator and acting asa total counter by integration of the fluid flow.

10. Apparatus according to claim 9, including an electrical supply andin which the counter has a current consumption equivalent to that of theheat compensating resistor and is supplied current by said supply inphase opposition to said heat compensating resistor in a fashion toassure the constant loading of said supply and in proportion to the flowof gaseous fluid.

1. Apparatus for counting and measuring the flow of a gaseous fluid in apipeline by measuring the quantity of heat necessary to raise thetemperature of a given quantity of the fluid, comprising a sensing meansdisposed in the gaseous flow and a reference means disposed in adead-end cavity containing the gaseous fluid in a manner to be sensitiveto fluid temperature but insensible to the flow of this fluid, in whicheach of the sensing means and reference means comprises a heat resistorfor continually heating the adjacent fluid in order to elevate itstemperature to a given quantity and a detection element sensitive to thetemperature, the sensing means comprising also a heat compensatingresistor, means responsive to a difference between the temperatures ofthe two detection elements for controlling the amount of current passingthrough said heat compensating resistor so as to cause the heatcompensating resistor to maintain the said elevation of the temperatureof the fluid flowing past said sensing means, and means for measuringcontinually the amount of current passing through said heat compensatingresistance.
 2. Apparatus according to claim 1, including a measuringhead having a body, in which the sensing means and the reference meansare both disposed in said measuring head, a tubular flow divider blockinserted into the pipeline through which flows the gaseous fluid forwhich it is desired to measure flow, said flow divider block being fixedto the measuring head body and communicating with the measuring head, onthe one hand by an entry orifice for applying fluid to the measuringhead and on the other hand by a discharge orifice by which the gas flowdirected into the measuring head rejoins the principle flow, the entryorifice being furnished with a calibrated port, a removable transverseplate provided with calibrated perforations located in the flow dividerblock between the entry orifice and discharge orifice.
 3. Apparatusaccording to claim 2, including means defining an annular chamber in thebody of the measuring head, the entry orifice of said directed gas flowopening into said annular chamber, said gas flowing in said annularchamber before reaching said sensing and reference means, the calibratedport associated with the entry orifice being very small so that thedirected gas flow is agitated to a turbulent movement in said chamber.4. Apparatus according to claim 3, including a hollow ring, thermallyinsulated, disposed in the interior of the annular chamber, so that thefluid takes the temperature of this ring, said ring having the samethermal phase shift as the probe of the sensing means.
 5. Apparatusaccording to claim 4, in which the cavity of the ring communicates atits lower portion with a first dead-end cavity in which is disposed thereference means and with a second similar cavity in which iS disposedthe sensing means, this second cavity also communicating at its upperportion with a conduit connected directly to the delivery orifice of thedirected gas flow.
 6. Apparatus according to claim 5, in which theinterior walls of the two cavities, in which are respectively disposedthe reference means and sensing means, are coated with a thermalinsulating material, such as plastic material.
 7. Apparatus according toclaim 1, including a measuring head incorporating said dead end cavityand further incorporating a through flow cavity, in which the referencemeans and the sensing means are essentially comprised of two identicalmetallic probes each in the form of a bell, said reference means probebeing located in said dead end cavity and said sensing means probe beinglocated in said through flow cavity, said bells each having a recesscontaining a metallic alloy into which are respectively received theheating resistor and the detecting element for the reference means, andthe heating resistor and the detecting element and the heat compensatingresistor for the sensing means, a provisional resistance, of the samenature as the heat compensating resistor of the sensing means, beingalso inserted into the recess of the probe of the reference means. 8.Apparatus according to claim 1 in which the two detecting elementssensitive to the temperature of the gas flow are comprised ofthermistors, said difference responsive means including means connectingsaid thermistors in two adjacent branches of a Wheatstone bridge, amonostable multivibrator supplying the heat compensating resistor of thesensing means, and means for amplifying the unbalance of the bridge andcontrolling the functioning of said monostable multivibrator, said meansfor continual measuring of the current passing through the heatcompensating resistor comprising a milliammeter and a filter circuitconnecting same in parallel with said heat compensating resistance. 9.Apparatus according to claim 8, including an impulse counter, controlledby the monostable multivibrator and acting as a total counter byintegration of the fluid flow.
 10. Apparatus according to claim 9,including an electrical supply and in which the counter has a currentconsumption equivalent to that of the heat compensating resistor and issupplied current by said supply in phase opposition to said heatcompensating resistor in a fashion to assure the constant loading ofsaid supply and in proportion to the flow of gaseous fluid.